The application of the principle of the coaxial injection of two liquid propellant components for hypergolic and non-hypergolic propellants has been known from U.S. Pat. No. 2,874,539. However, the simple, highly schematic representation is unable to provide the person skilled in the art with specific information for the design embodiment of the injection head or the injection elements. Especially hypergolic propellants must be absolutely separated from one another prior to entry into the intended reaction zone in order to avoid unintended reactions with possibly catastrophic consequences. Such safety aspects have not been taken into account in the design according to the prior art U.S. patent, either.
German Patent No. DE-PS 34 24 225 discloses injection elements for non-hypergolic propellants (e.g., H.sub.2 O.sub.2, CH.sub.4 /O.sub.2), which have proved to be highly successful especially in the design according to FIGS. 1 and 2. According to the coaxial principle, each injection element consists of a central body for the oxidant and of a two-part, screwed-on sleeve for the propellant, which sleeve is concentric to the central body and axially projects beyond the outlet of the central body. The outlet of the central body is conically expanded, has a sharp opening edge and twisting surfaces in the conical area. The sleeve has radial inlet holes and an essentially cylindrical, central channel, which surrounds the outlet of the central body. The known advantages of this coaxial arrangement are as follows:
a) Engines of different thrust classes can be manufactured with a standard injection element by varying the number of elements. PA1 b) Low thermal load on the injection head front plate, and consequently long service life. PA1 c) Low injection pressure drop, and consequently low tank or delivery pressure (gas pressure or pump pressure).
The mixture formation and combustion take place essentially consecutively in the case of non-hypergolic operation. The mixing of the usually gaseous propellant with the liquid oxidant is facilitated by an essentially higher velocity of admission of the propellant (velocity ratio, e.g., 15:1).
In the case of hypergolic propellants, the two components meet one another in the liquid form after the injection and react spontaneously with one another on contact. It is therefore important to provide for the largest possible reacting surfaces, i.e., to adequately prepare the two components separately from one another, and then to unite them without great differences in velocity.
An injection element of coaxial design for rocket combustion chambers with a combustion chamber pressure of 5 to 25 bar and for operation with two hypergolically reacting propellants has been known from German Patent No. DE-PS 38 18 623; the central body of this injection element for the oxidant has a twisting body as well as a conically expanding outlet with a sharp opening edge. The central body is surrounded in the outlet area by a concentric sleeve for adding the fuel, and this sleeve axially projects beyond the central body toward the combustion chamber to form a pre-reaction space. Specific limit values are mentioned for the geometry of the pre-reaction space as well as for the velocity ratios in its area. Practical experiments have unfortunately shown that this design tends to generate vibrations of the combustion chamber. This is explained by periodic microexplosions in the area of the pre-reaction space, which briefly interrupt the flow of propellant. Aside from the loss of power caused by this, the variations in pressure thus generated may destroy the engine structure in the worst case.